"One expected benefit of using legumes as a cover crop is to provide a source of nitrogen (N) to the cropping system. However, when legumes are included in mixtures with grasses and broadleaves for a relatively short growing period, the amount of actual fixed N may be relatively low. Even when planted as a monoculture or legume-only mixture, the amount of fixed N may be lower than anticipated."http://agfax.com/2016/12/19/nebraska-cover-crops-is-nitrogen-fixation-oversold-with-legumes/